The present invention refers to a driving control system for an electric motor, particularly a three-phase motor with permanent magnets, of the type used in refrigeration compressors and in which a control unit commands the adequate energization to the motor, as a function of the information received from a position sensor of this system operatively connected to the motor.
Brushless direct current three-phase motors are of interest in applications in which reliability and high efficiency are required.
Such motors consist of a stator containing coils, a rotor with permanent magnets, an inverter applying current to the stator coils, a position sensor, which informs on the time and period each coil assembly has to remain energized, and a central control, which processes the information on the current, rotor speed and rotor position and sends command signals to the inverter.
Three-phase motors with permanent magnets of the type used in compressors of refrigeration systems are electronically actuated and usually use inverters with three commutation arms. In this construction, each motor phase is connected between two commutation switches of a respective commutation arm of the inverter, the three arms forming this inverter being connected to the same power source through a rectifying diode bridge connected to a ripple filter capacitor.
In the actuation of motors with permanent magnets, it is necessary to detect the position of the motor, which may be achieved by sensors physically coupled to the rotor (hall effect, optic sensors, etc.) or by observing the voltages induced to the own coils of the motor, as described in Patent PI8805485 (U.S. Pat. No. 4,978,895). Detection of the rotor position allows the control system to select which motor phases will be actuated at each time.
The torque control may be usually effected in two ways: by current hysteresis or by controlling the voltage applied to the motor.
In the current hysteresis control, a median current is applied to the motor phases selected by a controller to be actuated. This current is controlled by a comparator with hysteresis which, when the current reaches a maximum value, turns off the selected switches, turning the latter on when the current reaches a minimum value and thus generating a voltage modulation signal (PWM), also called switching frequency (which is much higher than the rotation of the motor) and which maintains the current around a reference value, which is adjusted to maintain a constant speed. In this control, the voltage modulation signal (PWM), which is generated by the comparator, may vary its switching frequency and its switch conduction period (xe2x80x9cduty cyclexe2x80x9d, ratio between the closed switch period and full switching period) at each time. This type of control reacts more rapidly to load disturbs, but is more complex to be implemented in microcontrolled systems, since it needs at least one A/D converter and a powerful microcontroller.
In the voltage control, a voltage value is applied to the motor phases which have been selected by the controller to be actuated. This voltage is modulated by the voltage modulation signal (PWM), also known as switching frequency, which is generated by a timer and whose average value is adjusted by the control unit, in order to maintain the desired speed. This voltage adjustment is achieved by switching (on and off) the switches selected by the control unit during the period in which the motor phase is fed with voltage from a power source. Thus, in order to vary the average voltage value on the motor, the switch conduction period (ratio between the closed switch period and full switching period) is varied.
If the actual motor rotation speed is lower than the desired speed, the voltage in the motor is increased. If the actual motor rotation speed is higher than the desired speed, the motor voltage is reduced. These variations in the voltage value are adjusted by changing the value of the switch conduction period of the switching frequency: an increase of said value leads to an increase in the voltage value, while a reduction of said value leads to a reduction in the voltage value in the motor. In this control, the voltage modulation signal (PWM) usually has a fixed switching frequency and a switch conduction period, which is adjusted at each turn. This type of control does not react rapidly to sudden load disturbs and is indicated to applications in which the load does not vary suddenly, such as in refrigeration systems. However, it is a simpler control to be implemented in microcontrolled systems.
One of the disadvantages of the prior art construction is the amount of components involved in the inverter, high cost and low reliability. Moreover, it requires a large installation area to place all the components (the six electronic switches and their command, driving and protection circuits).
Thus, it is an objective of the present invention to provide a driving control system for an electric motor, particularly a three-phase motor with permanent magnets of the type used in refrigeration compressors, with a more compact, simpler and low cost construction.
This and other objectives are attained by a driving control system for an electric motor with N coils, N being at least three, including: a power source in direct current; an inverter having commutation arms with the opposite ends connected to the power source, each including switch means, each commutation arm being medianly connected to a respective coil; a position sensor means connected to the coils; and a control unit, operatively connected to the position sensor means and to the switch means, in order to operate the latter as a function of the signals received from the position sensor means. The inverter has Nxe2x88x921 commutation arms and the power source has, between positive and negative terminals of equal voltage, a median terminal, of null voltage, to which is directly connected a first coil, the inverter having a first switching condition of the switch means, in which the first coil is subjected to a null voltage, while two other coils are subjected to a determined voltage value, and a second switching condition of the switch means, in which the first coil and any of said other coils are subjected to a voltage value equivalent to half the determined voltage value and corresponding to the nominal voltage, the control unit determining to the switch means, the latter being in one of the first and second switching conditions, a switching frequency and a switch conduction period, which are defined so that the voltage value which is effectively applied to the coils is that one corresponding to the speed and torque required to the electric motor, independently from the switching condition of the switch means.